1. Field of the Invention
The present invention is related to the field of telephone set features and apparatus, and specifically those directed at reducing or eliminating handset feedback.
2. Description of the Related Art
It is not uncommon for currently available telephone sets to be equipped with volume controls for increasing or decreasing the amount of gain applied to the receiver circuit of the telephone handset. Such controls are added to accommodate the hearing impaired, to overcome excessive background noise, as well as to compensate for variances in transmission quality from one telephone call to the next. They are generally limited to adding 12 db or less of gain, so as to minimize the chance that the signal fed to the handset receiver will be introduced to the handset transmitter, resulting in the creation of a loud and unpleasant tone being sent through the receiver. This tone, caused by a feedback loop established between the receiver and transmitter of the handset, is commonly referred to as singing, but has also been referred to as howling, squealing, screeching, or simply feedback. However, even at the presently practiced limit of 12 db of receiver gain the potential for singing exists, depending on the impedance of the telephone line and the acoustic environment surrounding the handset.
As is well known in the art, the feedback loop which causes singing is created in part by acoustic coupling between the handset receiver in the handset earpiece and the handset microphone in the handset mouthpiece. The feedback loop is completed by electrical coupling between the transmit and receive circuits in the telephone hybrid.
As is well known in the art, telephone signals to and from a telephone are carried through two wires, long known in the telephony field as tip and ring. Within the telephone set these two wires connect to the telephone hybrid. The hybrid splits the tip and ring into a four wire circuit, two wires of which pass the far end caller's signal from the hybrid to the handset receiver as a receive signal. The remaining two wires carry a transmit signal from the handset transmitter to the hybrid for transmission over the tip and ring to the far end caller.
A perfect hybrid would result in no signal component leakage between the transmit and receive circuits of the hybrid, i.e., perfect sidetone cancellation or infinite trans-hybrid loss. In practice, trans-hybrid loss is never infinite and, instead, depends on how close the hybrid impedance network matches the actual line impedance of the phone line to which the telephone apparatus or set is connected. Since, in general, telephone line impedance is complex and is not known a priori by the telephone set, trans-hybrid loss is usually only about 6 db to 10 db. This means that a reduced amplitude version of the transmit signal will always be present in the receive signal. If the sum of the acoustic coupling, electrical coupling through the hybrid, and circuit gains exceed unity, the telephone will sing or howl.
For example, if the line impedance of the telephone line to which the phone is connected represents a significant impedance mismatch for the hybrid, and the handset is placed on a hard, acoustically reflective surface such as a smooth desk or countertop, the gain in this loop may exceed unity at some frequencies, causing singing to occur.
Moreover, this problem cannot be overcome by simply reducing the gain of the transmit signal when the receive signal gain is increased, which would merely reduce the volume of the speech heard at the far end by the other caller.
The present invention is directed at overcoming the aforementioned shortcomings found in presently known telephone sets.